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Bureau of Mines Information Circular/1987 



Roof and Rib Fall Accident and Cost 
Statistics: An In- Depth Study 



By Deno M. Pappas 




UNITED STATES DEPARTMENT OF THE INTERIOR 




Information Circular 9151 

ii 



Roof and Rib Fall Accident and Cost 
Statistics: An In- Depth Study 



By Deno M. Pappas 



UNITED STATES DEPARTMENT OF THE INTERIOR 
Donald Paul Hodel, Secretary 

BUREAU OF MINES 

David S. Brown, Acting Director 




Library of Congress Cataloging in Publication Data: 






Pappas, Deno M. 

Roof and rib fall accident and cost statistics: an in-depth study. 

(Bureau of Mines information circular ; 9151) 

Bibliography: p. 19-20. 

Supt. of Docs, no.: I 28.27: 9151. 

1. Mine roof control. 2. Mine accidents. I. Title. II. Series: Information circular (United 
States. Bureau of Mines) ; 9151. 



TN295.U4 



[TN317] 



622 s [363. 1' 19622] 



87-600117 



CONTENTS 



Abstract 

Introduction 

Acknowledgments 

Evaluation of roof-rib accident statistics 

Review of accident fatalities, 1910-84 

U. S. accident rates 

State accident rates 

Characteristics of U.S. roof-rib fall accidents 

Seasonal patterns 

Daily patterns 

Mine attributes 

Seam height 

Mine size 

Underground location 

Mining method 

The accident victim 

Worker activity 

Lost workdays 

Type of injury 

Cost analysis of roof -rib fall accidents 

Total accident costs 

Cost per accident 

Comparison of accident costs based on degree of injury 

Summary 

References. 

ILLUSTRATIONS 

1. Underground fatality statistics, 1910-84 

2. Roof-rib fall fatality statistics, 1931-84 

3. U.S. roof-rib fall accident rates, 1980-84 

4. U.S. roof-rib severity rates, 1980-84 

5. Roof-rib accident rates, by State 

6. Severity rates for roof -rib accidents, by State 

7. Average number of roof-rib accidents with respect to month of occurrence. 

8. Average number of roof-rib accident occurrences, with respect to time of 

day 

9. Roof -rib accident rates with respect to seam height of the mine 

10. Roof-rib accident rates with respect to mine size based on the average 

number of mine employees 

1 1. Roof -rib accident locations 

12. Frequency of roof-rib accidents with respect to type of mining method.... 

13. Average number of lost workdays due to roof-rib accidents 

14. Pie chart of roof-rib accident cost percentages, averages for 1980-84.... 

15. Comparisons of costs per accident 



Page 



1 


2 


3 


3 


4 


5 


6 


6 


6 


9 


9 


10 


11 


12 


12 


13 


13 


14 


14 


15 


15 


16 


17 


18 


19 



4 
4 
5 
5 
7 
8 
9 

9 
10 

11 
12 
12 
14 
16 
17 



11 






TABLES 



1. Roof -rib accident rates, longwall versus all other mining methods 

2. Worker activity roof-rib accident rates, averages for 1980-84 

3. Worker activity roof-rib fatality rates, averages for 1980-84 

4. Distribution of roof -rib accidents with respect to days lost, averages for 

1980-84 

5. Comparison of roof-rib accident costs and all other types of accident costs 

for 1 984 

6. Comparison of costs per roof-rib accident based on degree of injury for 

1983 



Page 

13 
13 
13 

14 

16 

18 





UNIT OF MEASURE 


ABBREVIATIONS USED 


IN 


THIS REPORT 


h 


hour 


MMst 




million short tons 


in 


inch 


st 




short ton 


min 


minute 


yr 




year 



ROOF AND RIB FALL ACCIDENT AND COST STATISTICS: 

AN IN-DEPTH STUDY 

By Deno M. Pappas 1 



ABSTRACT 

The purpose of this Bureau of Mines study of U.S. roof and rib (roof- 
rib) accident statistics and related accident costs is to define current 
accident trends (1980-84) associated with fatal and nonfatal roof-rib 
fall accidents. Data were retrieved from a data base containing all re- 
corded U.S. mining accidents, then sorted and normalized utilizing a 
computer software program. The statistics indicate that roof -rib acci- 
dents have significantly declined in the 5-yr study period. Moreover, 
they indicate that there have been increases and/or patterns of roof -rib 
accidents associated with specific mine characteristics, such as seam 
height, mine size, geographic location, and seasonal variations. Also, 
roof-rib injury characteristics produced pattern changes involving 
worker activity, lost workdays, types of injury, and severity of injury. 
A conservative estimate indicates that there has been a 30% adjusted in- 
crease in the cost of a roof -rib accident over the 5-yr study period. 

'Research civil engineer, Pittsburgh Research Center, Bureau of Mines, Pittsburgh, 
PA. 



INTRODUCTION 



Underground coal mining has always been 
recognized as a hazardous occupation. 
Between 1931 and 1984, over 29,000 coal 
miners lost their lives in underground 
accidents. This represents an average of 
one fatality for every 100,000 h worked 
underground. Over 50% of these 29,000 
fatalities were associated with roof-rib 
fall accidents; this is a greater amount 
than for any other class of accident. 
Although there have been articles written 
analyzing accident statistics, there has 
been very little written specifically on 
roof-rib fall accidents and costs. Such 
is the intent of this paper. 

Before looking at the statistics, the 
term "roof-rib fall" needs to be defined. 
Whenever an entry is developed in a coal 
seam, the surrounding coal and rock mass 
of the opening are no longer in equilib- 
rium. The rock mass in the roof has lost 
support from below, the floor rock no 
longer has an applied load from above, 
and the coal seam is no longer con- 
strained along the sides (rib and face) 
of the opening (1_). 2 If the roof is not 
adequately supported, the surrounding 
rock and coal may collapse or fall into 
the entry and randomly strike underground 
workers. In cases where an accident re- 
sults from the fall of the face, it will 
be considered a rib fall. Specifically, 
the failure of the roof or rib may be at- 
tributed to one or more of the following 
factors: 

1. Geologic anomalies. — Occur in the 
roof or rib as faults, slips, joints, 
rolls, clay veins, kettlebottoms , and 
sand channels. 

2. Effects of weathering. — Humidity 
or temperature changes may cause the 

^Underlined numbers in parentheses re- 
fer to items in the list of references at 
the end of this report. 



roof to deteriorate, which leads to roof 
failure. 

3. Stress conditions. — Stresses occur- 
ring within the strata because of the ef- 
fects of the overburden or the effects of 
past geologic activity may result in a 
roof-rib fall. For example, bumps or 
bursts result in a sudden and violent 
rupture of the supporting coal pillars 
because the vertical unit loading of the 
pillar exceeds the bearing strength of 
the coal (2). High horizontal stress is 
another stress condition that may cause a 
roof-rib fall. The horizontal stress may 
be in excess of the vertical stress, re- 
sulting in open cracks along the entries 
that may lead to failure of the roof. 

4. Mining method. — The method employed 
may initiate roof-rib failure such as in 
longwall mining and retreat room-and- 
pillar mining. 

5. Scaling. — The occurrence of roof 
failure caused by a worker barring down 
the roof. Although, the failure of roof 
was initiated by the worker, the accident 
is still classified as a roof fall. 

Most often roof-rib fall accidents 
involve one or more of the preceding 
factors, coupled with the fact that the 
victim was under unsupported roof near 
the face. The working face area is the 
most hazardous area because the stresses 
are being actively redistributed and 
failure can occur instantly. Not in- 
cluded as roof-rib falls are accidents 
caused by haulage equipment knocking out 
roof support because it is the motion of 
the machinery that causes the accident. 
Therefore, roof-rib falls can be related 
to the unpredictable behavior of a rock 
mass in transition from one state of 
equilibrium to another and may be 
initiated by several factors. 



ACKNOWLEDGMENTS 



The author gratefully acknowledges Mrs. 
Betty J. Hamilton, computer programmer 
analyst, Theoretical Support Group, 



Pittsburgh Research Center, for direction 
in the use of the various computer soft- 
ware packages. 



EVALUATION OF ROOF-RIB ACCIDENT STATISTICS 




To evaluate the roof-rib accident data, 
it is first necessary to define the mean- 
ing of the term "accident." For this 
study, an accident is defined as any mis- 
hap that results in a fatal or nonfatal 
injury, including injuries that do not 
result in lost workdays. It is important 
to include nonfatal accidents data along 
with the fatal accidents to obtain a 
larger data base and a more complete pic- 
ture of the extent of roof-rib fall acci- 
dents. For every roof-rib fatality, 30 
nonfatal accidents occur that result in 
over 39,000 lost workdays (averaged) an- 
nually. Therefore, to get a more com- 
plete data base, the statistics include 
both fatal and nonfatal roof-rib fall ac- 
cidents for the 1980-84 period, except 
where otherwise noted. 

The following five types of accident 
rates were calculated to evaluate roof- 
rib accidents in U.S. underground coal 
mines. These rates normalize the acci- 
dents with respect to controlling factors 
such as hours worked underground, produc- 
tion, and size of work force, to give 
a more concise measure of roof-rib 
accidents. 

1. Roof-rib fall accidents per 200,000 
employee-hours worked underground. The 
200, 000-h figure approximates the number 
of hours worked by 100 full time miners 
per year. 

2. Roof-rib fall accidents per million 
short tons of underground coal produced. 

3. Roof-rib fall accidents per average 
total number of underground workers. 
This rate is actually a percentage of the 
average underground work force injured in 
these accidents. 

4. Total number of lost workdays due 
to roof-rib accidents per 200, 000 



employee-hours worked underground. This 
rate is known as the nonfatal severity 
rate and indicates the seriousness of 
nonfatal accidents. 

5. Total number of lost workdays due 
to roof-rib fatal, nonfatal, and perma- 
nently disabling accidents per 200,000 
employee-hours worked underground. This 
rate is referred to as the overall 
severity rate. It is similar to the non- 
fatal severity rate except that it ac- 
counts for accidents resulting in a fa- 
tality or a permanent total disability. 
Permanently disabling and fatal injuries 
are each charged 6, 000 days (3)» 

Raw data for this study were obtained 
with the use of the Health and Safety 
Analysis Center (HSAC) accident file 
(from the Mine Information Systems of the 
Denver Safety and Health Technology 
Center of the Mine Safety and Health 
Administration (MSHA)), which has on rec- 
ord all reported U.S. mining accidents, 
and was retrieved with the use of the Bu- 
reau's program (HDBSEL). The records for 
all roof-rib accidents that resulted in 
fatal or nonfatal injuries (degree 1 to 
6) were retrieved and transferred to the 
RS/1 (Research Systems 1) software pack- 
age designed by BBN Research Systems.-^ 
The use of RS/1 permitted the retrieved 
records to be tabulated, sorted, and 
graphed. 

Certain factors may be related to the 
increase or decrease of accidents. How- 
ever, an exact list of the factors in- 
fluencing a pattern of roof-rib accidents 
is nearly impossible to compile. Equally 

•^Reference to specific products does 
not imply endorsement by the Bureau of 
Mines. 



difficult is the task of proving unequiv- 
ocally that the factor is associated with 
the accident statistics. There are many 
hidden factors that may additionally af- 
fect the compiled data such as lengthy 
strikes, catastrophic disasters, and in- 
consistencies in recording accident in- 
formation. The accuracy of this study is 
only as accurate as the accident records 
entered into the data base. Consequent- 
ly, this study emphasizes accident rate 
trends that occurred over the 5-yr study 
period rather than specific accident num- 
bers or rates. Factors affecting acci- 
dent trends are suggested, but are not 
necessarily the only factors involved 
and, in most instances, cannot be 
definitely confirmed. 

REVIEW OF ACCIDENT FATALITIES, 1910-84 

A review of accident fatalities (fig. 
1) over the past 74 yr reveals a dramatic 
drop (approximately 96%) in the total un- 
derground fatalities as well as in roof- 
rib fall fatalities. Some of the major 
reasons for these decreases are as 
follows: 

1. Decreases in accidents between 1920 
and 1950 seem to coincide with drops in 
production due to slowdowns in the econo- 
my; e.g., the Great Depression of the 
1930's, and the post-World War II reces- 
sion of the late 1940's. 

2. Mechanization of the mining indus- 
try during the early 1950's, considerably 
improved productivity and required a 
smaller work force. As the number of 
employee-hours decreased, so did the num- 
ber of accidents (4^). 

3. The increased use of roof bolts 
starting in the 1950's. 

4. Federal legislation, especially the 
1952 Federal Coal Mine Act and the 1969 
Coal Mine Health and Safety Act, which 
promoted underground federal inspections 

5. Bureau of Mines research efforts in 
developments of the ATRS (automated tem- 
porary roof support), FOPS [falling ob- 
ject protective structures (canopies)], 
methane drainage techniques, permissibil- 
ity criteria for explosives, etc. 



3,000 

2,500 

w 2,000 

5 1,500 

< 1,000 

500 



KEY 

Total underground fatalities 

Roof- rib fatalities 

Roof-rib fatalities, % 







M L-'iA 



100 
80 
60 I 



40§ 

o 

< 

20 



1900 1920 1940 I960 1980 2000 
FIGURE 1.— Underground fatality statistics, 1910-84. 

KEY 
Roof-rib fatalities 



Fatality rate : roof- rib accidents 

per 200,000 employee- hours 
• i i i I i i i 




I930 I940 I950 I960 I970 I980 I990 
FIGURE 2. — Roof-rib fall fatality statistics, 1931-84. 

6. A greater awareness of underground 
hazards promoted by mandated safety 
training and supplemented by the union 
and MSHA safety programs such as REAP 
(roof evaluation accident prevention). 

To obtain a more accurate picture of 
the rate of roof-rib fall fatalitites, 
the data were normalized, based on the 
total employee-hours worked. From figure 
2 it is quite clear that although the 
number of fall fatalities started to 
decrease in the 1940's, it was not until 
the 1960's that the fatality rate (based 
on employee-hours worked) actually 
started to drop. Examination of roof-rib 
accident percentages (fig. 1) shows that 
a consistently high percentage (50%-70%) 
of all the underground fatalities re- 
sulted from roof-rib falls. It was only 
recently that these percentages moderate- 
ly dropped. During the early 1980's, 
roof-rib fall fatalities averaged about 



40% of all underground fatalities, which 
is still greater than any other type of 
accident. 

U.S. ACCIDENT RATES 

Reviewing the various U.S. accident 
rates (fig. 3) illustrates that roof-rib 
accidents have consistently decreased 
over the last 5 yr, hitting a new low 
rate in 1983 and rebounding somewhat in 
1984. These accident rates all confirm 
that a decrease in roof-rib accidents oc- 
curred: Accidents per million short tons 
mined dropped 42%, accidents per 200,000 
employee-hours worked decreased 17%, and 
accidents per average number of workers 
dropped 15%. 

A probable reason for the decrease may 
be due to the fact that between 1980 and 
1984 there was a 28% decrease in the 
average number of employees working un- 
derground (_5). Even though the rates are 
normalized linearly, the effects of a 
smaller work force may have a dispropor- 
tional effect in decreasing the number of 
roof-rib accidents. Possibly, the mining 
companies elected to keep their more ex- 
perienced employees, which may have 
resulted in a higher concentration of 
safety orientated miners. 

It is interesting to note that although 
the number of workers decreased 28%, un- 
derground coal production (short tons 

KEY 
• Based on production 
a Based on employee-hours worked 
■ Based on total number of workers 



< 



a> 

° o 
a. _c 



1.4 



1.0 



0.012 



.008 



.004 







1 


A - 






• 






- 




. 






B - 


- 




" 






■ 










C - 


- 


1__ 


1 ' 





1980 



1981 



1982 



1983 



1984 



FIGURE 3.— U.S. roof-rib fall accident rates, 1980-84. A, ac- 
cidents per million short tons of underground coal mined; B, 
accidents per 200,000 employee-hours worked; C, accidents 
per average total number of underground workers. 



produced per year) has increased 6. 3% and 
underground coal productivity (short tons 
produced per 8-h shift) has increased 41% 
over the same time span 05). Recently, 
Spokes (6) found a correlation between 
declining accident rates and increasing 
productivity. However, this does not 
necessarily mean that increasing the pro- 
ductivity will definitely cause fewer ac- 
cidents; there are many other factors 
intertwined. 

Examining the severity rates in figure 
4 does not show any definite trends ex- 
cept that both severity rates have errat- 
ically increased over the 5-yr study 
period. The overall severity rate 
increased approximately 30%, and the non- 
fatal severity rate increased less than 
7%. This is a possible indication that 
the seriousness of injuries resulting 
from roof-rib fall accidents is on the 
increase. 



v> 




1980 1981 1982 1983 1984 



FIGURE 4.— U.S. roof-rib severity rates, 1980-84. A, number 
of days lost (related to nonfatal accidents) per 200,000 
employee-hours worked; 8, number of days lost and days 
charged (related to fatal and permanent total disability ac- 
cidents) per 200,000 employee-hours worked. 



STATE ACCIDENT RATES 

Examination of equivalent accident 
rates by State points out some regional 
differences. Figures 5 and 6 show the 
roof-rib fall accident rates for the top 
10 underground coal producing States. 
The following comparisons were drawn from 
these figures and are compared with the 
national rate. 

Most noticeable in all of the bar 
charts are the considerably higher roof- 
rib fall accident rates in the Western 
States of Utah and Colorado. On the 
average, 1.8% of Utah's and Colorado's 
work force was injured in roof-rib fall 
accidents alone, as compared with the 
national average of 0.9% (fig. 5c). This 
may be due to several factors uniquely 
associated with western coal mines. 
These include abnormal seam characteris- 
tics (deeper, thicker, and pitching 
seams) and a less experienced work force. 
In the east, the State of Virginia also 
had fairly high accident rates (fig. 5). 
However, there was a downward trend in 
these higher accident rate States. 

During the early part of the study, 
Kentucky had the lowest roof-rib fall ac- 
cident rates in the country but, by the 

CHARACTERISTICS OF U.S. 

The analysis of the compiled statistics 
included detailed examination of roof-rib 
accident characteristics related to the 
time of occurrence, to specific mine at- 
tributes, and to the accident victim. 
Within each grouping, all available data 
on each roof-rib accident were compiled 
and evaluated. The data measured fre- 
quency or rate of occurrence to emphasize 
particular trends within each category. 

SEASONAL PATTERNS 

Data on the occurrence of roof-rib 
falls with respect to time of year were 
compiled to determine if seasonal 



end of the study period, Tennessee, Ala- 
bama, and Pennsylvania had equal or some- 
what lower accident rates. Figure 5 in- 
dicates that Kentucky's accident rates 
were gradually approaching the national 
rate, while rates of most other States 
were on the decline. 

Between these two extremes of accident 
rates were Alabama, Illinois, Tennessee, 
and West Virginia, which hovered around 
the national rate (fig. 5). It is inter- 
esting to note that West Virginia's acci- 
dent rates closely followed the national 
rates. Pennsylvania's and Ohio's acci- 
dent rates, which were fairly high ini- 
tially, dropped consistently after 1982 
(fig. 5). 

Severity rates in Tennessee, Virginia, 
and the Western States were very high 
initially, but then dropped or stabilized 
(fig. 6). In a reverse situation, Ken- 
tucky's rates were fairly low initially, 
but then increased significantly above 
the national average (fig. 6). Ken- 
tucky's high overall severity rate in 
1984 is due in part to the disproportion- 
ate number of roof-rib fatalities (20 
fatalities) that occurred that year. The 
remaining States fall close to or below 
the national severity rate. 

ROOF-RIB FALL ACCIDENTS 

patterns such as fluctuations in tempera- 
ture, barometric pressure, and humidity 
might affect the frequency of roof-rib 
falls. Figure 7 displays the average 
number of roof-rib fall accidents 
(between 1980 and 1984) with respect to 
month of accident. Data from 1981 were 
omitted because it was a strike year and 
monthly accident results were biased. 
Also, roof-rib accidents that occurred in 
mines of the Western United States (Utah, 
Colorado, New Mexico, and Wyoming) were 
omitted because these areas are mostly 
arid climates and experience minimum 
fluctuations in humidity. 



ALABAMA [ 

COLORADO 

ILLINOIS 

KENTUCKY 

OHIO 

PENNSYLVANIA 

TENNESSEE 

UTAH 

VIRGINIA 

WEST VIRGINIA 

UNITED STATES 



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E3I98I 
□ 1982 

E3I983 
EUl 1984 



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2 4 6 8 10 12 14 04 0.81.2 1.6 2.024 28 3.2 3.6 

RATE, accidents per — 



0.01 



0.02 



0.03 



MMst 



200,000 
employee -hours 



Worker 



FIGURE 5.— Roof-rib accident rates, by State. A, accidents per million short tons of underground coal mined; B, accidents per 
200,000 employee-hours worked; C, accidents per average total number of underground workers. 



Although it was not possible to normal- 
ize the results based on employee-hours, 
there did seem to be a seasonal trend. 
The number of roof-rib accidents peaked 
in the months of August through October 
and then dropped off during the months of 



November through February. This trend 
may be due to an increase in coal produc- 
tion during the late summer for winter 
stockpiling, resulting in an increase in 
underground exposure time and subsequent- 
ly more accidents. Because monthly 



ALABAMA 



COLORADO 




,200 



RATE, days lost per RATE, days lost and 
200,000 charged per 200,000 

employee-hours employee -hours 

FIGURE 6.— Severity rates for roof-rib accidents, by State. A, days lost (related to nonfatal accidents) per 
200,000 employee-hours worked; B, days lost and days charged (related to fatal and permanent total disability ac- 
cidents) per 200,000 employee-hours worked. 







50 



y>* *e* ^ ^ ^ y»* ^ ^ <b& 0^ ^ O eC 
MONTH 

FIGURE 7.— Average number of roof-rib accidents with 
respect to month of occurrences (excludes Western States). 



production figures are unavailable, 
this speculation could not be verified. 

However, a similar seasonal trend of 
roof falls was reported by Stateham and 
Radcliffe (_7), who found that humidity 
has a strong influence on roof fall oc- 
currence rates. Using cubic regression 
techniques, they correlated humidity and 
roof fall statistics over a 3-yr period 
and found that their best-fit curves al- 
most coincided in sinusoidal cycles. The 
roof fall occurrence rate curve followed 
the humidity curve by about 14 days. 
Their results indicated that the proba- 
bility of a roof fall is greatest in Au- 
gust and least in February. Their study 
also indicated that the barometric pres- 
sure was not related to roof falls. 

Another study by Haynes (8) found that 
the effect of temperatures and tempera- 
ture changes on rock around mine openings 
has a negligible effect on roof stabil- 
ity. Consequently, it appears that the 
only climate condition that may play a 
part in the occurrence of roof-rib falls 
is humidity. 

DAILY PATTERNS 

Another time-related parameter focuses 
on the approximate hour at which the 
roof-rib fall accident occurred. Figure 
8 shows the number (5-yr average) of 
roof-rib accidents within 30-min inter- 
vals during a 24-h period. The peak num- 
ber of accidents occurred between 10:00 
and 10:30 a.m., 1:00 and 1:30 p.m., and 
6:00 and 6:30 p.m. The 10:00-10:30 a.m. 
peak may coincide with the ap-proximate 
time of day miners are in full operation 
at the face; however, by noon the number 
of accidents has dropped by half, 



i i i | i I i | ■ i i | r i i | i ' ' 1 ' ' ' I ' ' ' I ' ' ' I ' ' ' I ' ' ' 1 ' ' ' I 




12 2 4 6 8 10 12 2 4 6 8 10 12 

a.m. noon ►p.m. 

30-MIN TIME INTERVALS 

FIGURE 8.— Average number of roof-rib accident occur- 
rences with respect to time of day. 



probably because of the lunch break. The 
1:00-1:30 p.m. peak may signify full 
operation after lunch, and the 6:00-6:30 
p.m. peak may be due to the evening shift 
in full operation. These peaks may indi- 
cate a pattern of more accidents occurr- 
ing in the second and third hour into the 
respective shifts and also after the 
lunch break. 

Theodore Barry and Associates (_9) found 
a similar pattern in a study conducted in 
the late 1960's. This study speculated 
that the higher number of roof falls in 
this period was due to lack of adequate 
testing of the roof at the start of a 
shift, since the miners, having just 
started their daily tasks, were not fo- 
cusing enough attention on the behavior 
of the roof. Figure 8 shows a low number 
of roof-rib accidents at the start and 
end of each shift, probably because the 
workers were in transition and away from 
the face areas. These accident trends 
can probably be related more to human 
factors than to the occurrence of roof- 
rib falls. 

MINE ATTRIBUTES 

According to the Department of Energy 
(10), there were over 1, 760 underground 
coal mines operating in the United States 
in 1984. Although no two mines are iden- 
tical, it was thought that common mine 
characteristics such as seam height, mine 
size, underground location mining method, 
etc., may be associated with the frequen- 
cy of roof-rib fall accidents. 



10 



Seam Height 

One of the obvious mine characteristics 
that was considered was the seam height. 
A scan of Figure 9A shows that roof-rib 
accidents occur at a higher rate for thin 
seams (<36 in) and for very thick seams 
(>120 in). Examination of the fatality 
rates (fig. 9B) also shows higher rates 
for thin seams; however, the higher rates 
do not extend to the thick seams. Figure 
9 shows that accident and fatality rates 
for mines with intermediate seam thick- 
nesses (37-120 in) are fairly low. Data 
for these figures cover only the 1983-84 
period because the seam height was incon- 
sistently reported for the other years. 

The somewhat higher accident rates for 
thick-seam mines, although not reflected 
in the fatality rates, may possibly be 
due to the higher roof, which allows any 



falling material to gain more velocity 
and thereby cause more serious injuries. 
Possibly, the more extensive use of 
canopies on mining equipment in thicker 
seams may limit the occurrences of a fa- 
tal roof-rib accidents. 

The higher accident rates for thinner 
seams (<36 in) may be due, in part, to 
the extremely confined work area. The 
low head room makes assessment of the 
roof difficult and inhibits escape from 
an impending roof fall. The thinner 
seams also limit the type of mining that 
can be used; for example, the use of 
longwall mining, which may provide better 
protection from roof-rib falls, is pre- 
cluded. Another possible explanation 
given by MSHA (11) is the lack of cabs 
and canopies on low-coal mining face 
equipment, resulting in less protection 
and a higher frequency of accidents due 



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FIGURE 9.— Roof-rib accident rates with respect to seam height of the mine. A, fatal and nonfatal roof-rib accidents per 200,000 
employee-hours worked; S, only fatal roof-rib accidents per 200,000 employee-hours worked. 



11 



due to roof falls. The higher accident 
rate may not be totally attributed to the 
seam height, it may also be interrelated 
with other factors such as the large num- 
ber of low-coal mines that are small 
mines. And as the succeeding section ex- 
plains, smaller coal mines have higher 
roof-rib accident rates. 

Mine Size 

The size of the mine was reviewed on 
the assumption that larger mines have 
larger technical staffs and more capital 
to deal with ground control problems. 
Small mines have a minimal technical 
staff (if any), little capital, and are 
sometimes located in unusual seams with 
difficult ground control problems. Con- 
sequently, the smaller mines may have a 
higher accident rate, as documented in 
accident studies conducted by the 
National Academy of Science and National 



Research Council (12-13). Their studies, 
which included all types of accidents, 
found a strong correlation between mine 
size and fatal injuries. 

Since the size of a mine can be quanti- 
fied by the magnitude of its work force, 
this Bureau study normalized all roof-rib 
accidents and fatal accidents based on 
the annual average total number of em- 
ployees (fig. 10). Examination of the 
accident rates (fatal and nonfatal) in 
figure 10A shows that, initially, inter- 
mediate-size mines (51 to 150 employees) 
had high accident rates, but by 1983 
these rates had dropped considerably. 
Over the same period, accident rates in 
small mines (1 to 20 employees) increased 
above all the other mine size categories. 
Even more pronounced is the fatal acci- 
dent rate (fig. 105), which shows that 
small mines have a significantly higher 
fatality rate than all other groups. It 
is interesting to note from both graphs 



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RATE, accidents per 
200,000 employee -hours 



0.04 0.08 0.12 0.16 0.20 
RATE, fatal accidents per 
200,000 employee -hours 



FIGURE 10.— Roof-rib accidents rates with respect to mine size based on the average number of mine employees. A, fatal and 
nonfatal roof-rib accidents per 200,000 employee-hours worked; B, only fatal roof-rib accidents per 200,000 employee-hours 
worked. 



12 



that very large mines (greater than 250 
employees) have the lowest roof -rib acci- 
dent rates of all categories, especially 
for fatal accidents. 

Underground Location 

The location of the roof-rib fall acci- 
dent was another factor studied. Fig- 
ure 11 shows that approximately 61% of 
roof-rib fall accidents occur at the 
working face, 12% at an intersection, and 
the remaining portion at various other or 
undefined locations. These results seem 
reasonable since most miners work at the 
face. At the face, the miners are often 
under temporarily supported or unsup- 
ported roof, where there is a much great- 
er risk of a roof fall. No trends from 
year to year were detectable with respect 
to location. 

Mining Method 

Further examination of mine attributes 
involved the mining method employed at 
the location where the accident occurred. 
Longwall mining is becoming a popular 
method of mining, which is evident by a 
100% increase in the number of working 
longwall faces in the United States be- 
tween 1974 and 1986 (14-15). The rapid 
growth in use of the longwall method may 
be attributed to a higher recovery rate 
and safer working climate, since the min- 
ers are almost always under the protec- 
tion of the longwall shields. 

Figure 12 is a compilation of the num- 
ber of roof-rib accidents based on the 
mining method used at the section where 
the accident occurred. The bar chart 
shows accidents decreasing for the con- 
ventional and continuous mining methods, 
with the number of longwall accidents in- 
creasing slightly with time. This slight 
increase is most likely due to the 
increased number (approximately 59 pan- 
els) of longwall panels started in the 
5-yr period. 

For a more accurate assessment of acci- 
dent trends, the data needed to be nor- 
malized based on annual tonnage per min- 
ing method. Table 1 shows the accident 
rates of the longwall method versus all 
other methods (conventional, continuous, 



Intersections 
12.5% 




Face 
61.3% 



Shaft, shop, 
slope 
3% 



Other or undefined 
23% 



FIGURE 11.— Roof-rib accident locations. 




KEY 

MI980 

□ I98I 

□ 1982 

□ 1983 
^1984 



Continuous 



Conventional 
MINING METHOD 



^^ZEfi 



Longwall 



FIGURE 12.— Frequency of roof-rib accidents with respect 
to type of mining method. 

and hand loading) grouped together and 
normalized with production figures from 
1978 and 1983, since these are the only 
years for which longwall production fig- 
ures were published ( 10 , 16). However, 
these rates are somewhat biased since 
longwalls are considerably more produc- 
tive than other mining methods and the 
number of accidents that the longwall 
rate is based upon is considerably less 
than the number of accidents for the 
other rate, which may affect the 
accuracy. 

Both longwall and the other mining me- 
thods show a substantial accident rate 
drop over the 5-yr time span (from 3.5 to 
1.48 accidents per million short tons for 
longwalling and 4. 88 to 2. 98 accidents 
for all other methods) reflecting the 
overall drop in roof-rib accidents. Yet, 
the percentage difference in roof-rib ac- 
cident rates between the longwall method 
and all other methods widened from 28% in 
1978 to 50% in 1983 (table 1). 

Although these data are somewhat 
biased, they may indicate that longwalls 
provide a safer working environment, as 
was found in an equivalent accident study 
conducted by Peake (17). Peake's study, 



13 



TABLE 1. - Roof-rib accident rates, longwall versus all other 
mining methods 



Production, st: 

Longwall 

Other methods 

Number of roof -rib accidents: 

Longwall 

Other methods 

Accident rate: ' 

Longwall 

Other methods 

Difference 2 %. 



1978 



1983 



11,981,000 


47, 


257 


,000 


217,094,592 


244, 


885 


,378 


42 






70 


1,061 






731 


3.50 






1.48 


4.88 






2.98 


28 






50 



Accidents per million short tons mined. 

Percentage by which roof-rib accident frequency for all 
other mining methods exceeded that for longwall mining in the 
United States. 



conducted in 1985, examined all types of 
accidents for 13 longwall and nonlongwall 
working faces and the data were normal- 
ized based on employee-hours worked in- 
stead of production. Peake's study found 
that the nonlongwall raining method acci- 
dent rate exceeded the longwall rate by 
53%. 

THE ACCIDENT VICTIM 

One of the more important concerns in 
compiling these statistics was to evalu- 
ate the characteristics and effects that 
a roof-rib accident has on the victims 
and their families. Various factors 
evaluated include the victim's activity 
at the time of the roof-rib accident, 
the number of lost workdays, and the type 
of injury resulting from the accident. 

Worker Activity 

The types of activities that the vic- 
tims were pursuing at the moment the 
roof-rib fall occurred were evaluated. 
Tables 2 and 3 show the top 10 worker ac- 
tivities, with the highest accident and 
fatality rates based on employee-hours 
worked (5-yr average). Handling sup- 
plies, barring down the roof-rib, and 
roof bolting had the highest accident 
rates (table 2). Examination of the fa- 
tality rates in table 3 shows almost the 
same worker activities as table 2 except 
in a different order, possibly indicating 



TABLE 2. - Worker activity roof-rib 
accident rates, averages for 
1980-84, accidents per 200,000 
employee-hours 



Rank 



1.. 
2.. 
3.. 
4.. 
5.. 
6.. 
7.. 
8.. 
9.. 
10. 



Worker activity 



Handling supplies or material 

Bar down 

Roof bolter, other 

Set, remove, relocate props.. 

Continuous miner 

Walking, running 

Idle 

Machine maintenance, repair.. 

Timbering 

Move power cable 



0.088 
.081 
.070 
.061 
.060 
.050 
.050 
.045 
.043 
.035 



TABLE 3. - Worker activity roof-rib 
fatality rates, averages for 
1980-84, fatalities per 200,000 
employee-hours 



Rank 



1... 
2... 

■J * • >> 

4... 
5... 
6. • • 
7... 
8. . . 
9... 
10.. 



Worker Activity 



Continuous miner 

Timbering 

Observe operations 

Roof bolter, inserting bolt 

Handling supplies 

Supervise 

Set, remove, relocate props 

Roof bolter, other 

Bar down 

Walking, running 



0.0035 
.0025 
.0022 
.0022 
.0021 
.0021 
.0019 
.0018 
.0017 
.0014 



which activities are more prone to result 
in a fatal accident. Operating the 



14 



continuous miner, timbering, and observ- 
ing operations had the highest fatality 
rates. Although many of the activities 
listed in table 3 correspond to activi- 
ties that usually occur at the working 
face, several of the activities were 
merely observing operations, supervising, 
walking, or idle time. These data empha- 
size the randomness at which a roof-rib 
fall can occur. 

Lost Workdays 

Most of the attention associated with 
roof-rib accidents is focused on the fa- 
talities, and rightly so. However, out 
of the average 1, 100 roof-rib accidents 
that occur each year, less than 4% of 
these accidents are fatalities. Whereas, 
over 50% of these accidents are severe 
enough to result in 10 or more lost work- 
days, as shown in table 4, which gives a 
cumulative percentage of days lost. 

MSHA defines days lost as the number of 
full calendar days that the injured em- 
ployee is unable to work as a result of a 
temporary disability (3). This does not 
include lost workdays from an accident 
resulting in a permanent total disabil- 
ity. Consequently, the statistics offer 
a relative indication of the seriousness 
of roof-rib accidents in terms of the 
number of lost workdays. 

Data from figure 13 indicat-e that the 
average length of time away from work be- 
cause of a roof-rib fall accident has in- 
creased from 35 to 44 days over the 5-yr 
study period. This indicates that the 
victim is requiring a longer recuperation 
period, and one reason may be that roof- 
rib accidents are becoming more severe, 
as was noted with the severity rates. A 



TABLE 4. - Distribution of roof-rib 
accidents with respect to days 
lost, averages for 1980-84 



Days lost 



>499 

249 to 499. 
143 to 249. 
99 to 143.. 
88 to 99... 
66 to 88... 
44 to 66. .. 
22 to 44... 
10 to 22... 

4 to 10 

1 to 4 



1 



Total. 



Av No. of 
accidents 



2 

19 

42 

42 

18 

49 

90 

155 

160 

176 

157 

148 

40 



1,098 



Cumulative 
percentage 



0.2 

1.9 

5.7 

9.5 

11.2 

15.6 

23.9 

37.9 

52.5 

68.5 

82.9 

96.3 

100.0 



NAp 



NAp Not applicable. 

'Fatal and other accidents resulting in 
permanent total disability. 

tally of the total workdays lost between 
1980 and 1984 yields almost 200,000 days 
or 1.6 million employee-hours lost be- 
cause of roof-rib accidents alone (These 
figures do not include workdays lost from 
an accident resulting in a permanent 
total disability. ) 

Type of Injury 

Analysis of the types of injuries sus- 
tained by roof-rib fall victims shows 
that 17% of the victims suffer injuries 
to multiple parts of their bodies. Mul- 
tiple injuries are most often linked with 
severe or fatal injuries. Following mul- 
tiple injuries, the most frequent 
injuries are to the back and the 




1980 1981 1982 1983 

FIGURE 13.— Average number of lost workdays due to roof-rib accidents. 



I984 



15 



extremities (finger, foot, hand, and 
leg). These injuries are usually less 
sever than multiple injuries. The parts 
of the body injured in roof-rib accidents 
seem fairly well represented because fall 
material injures susceptible areas of 
the body such as the back, extremities, 
etc. In many instances, injuries affect 
more than one area. 



Roof-rib accidents not only affect the 
victim physically but also financially. 
This introduces a new factor: What eco- 
nomic effects do roof-rib accidents have 
on their victims, as well as on the 
mining industry? 



COST ANALYSIS OF ROOF-RIB FALL ACCIDENTS 



To evaluate the economic effects that 
roof-rib fall accidents cause, the com- 
puter software package ACIM (accident 
cost indicator model) was utilized. This 
cost analysis program, developed under 
contract for the Bureau, was mathemati- 
cally modeled to incorporate information 
gathered from mine inspection offices, 
workmen's compensation agencies, insur- 
ance companies, and major medical cen- 
ters. Output from the program estimates 
the tangible costs, both total cost and 
cost per accident, for a specified type 
and degree of accident. The costs are 
broken down into the following categories 
( 18-19 ) : 

1. The cost of the accident to the 
mining industry. — This includes the cost 
of cleanup after the accident and associ- 
ated production losses, the cost of State 
Worker Compensation Benefits, the cost of 
the medical treatment, the cost of the 
union death-disability benefits, and the 
cost of the mining industry's investiga- 
tion of the accident. 

2. The cost of the accident to the 
victim and victim's family. — Cost due to 
lost wages during recuperation or the 
lost wages for the remaining portion of 
the victim's career if the accident re- 
sults in a fatality or permanent (total) 
disability. Lost wages are adjusted to 
account for compensation wages received 
from benefits. 

3. The cost of the accident to the 
public sector. — This includes the cost of 
benefits (Federal Social Security) paid 
out to the victim and victim's family and 
the cost of the public investigation con- 
ducted by MSHA. 



These costs are based on cost data for 
a particular year and are not adjusted 
for inflation unless otherwise stated. 
Also excluded are costs of lawsuits; 
costs of hiring and training replacement 
workers; costs of resarch to prevent or 
reduce accidents; and most costs related 
to lost profit, sales, or equipment idled 
by an accident (18). 

It should be emphasized that the ACIM 
program does not use the actual costs 
associated with each accident but rather 
a random generator to approximate the ac- 
cident costs. Therefore, these costs 
should be used cautiously since they are 
only estimated costs of roof-rib acci- 
dents. This is especially true for fatal 
accidents, where no true cost can be 
placed on the loss of human life. 

TOTAL ACCIDENT COSTS 

The software program approximates the 
total cost of all roof-rib accidents be- 
tween 1980 and 1984 at $215 million or 
about 27% of the cost of all underground 
accidents. A breakdown of the groups of 
individuals economically affected by 
roof-rib accidents (averaged 1980-84) is 
shown in the pie chart in figure 14. The 
major costs of these accidents between 
1980 and 1984 are carried by the mining 
industry (approximately 47% or $100 mil- 
lion), the victim and victim's family 
(approximately 32%, or $70 million), and 
public agencies (approximately 21% or $45 
million). Also shown in figure 14 is the 
breakdown of the cost percentage involved 
within each grouping. The major roof -rib 
accident costs result from production 
losses ($37 million), lost wages ($70 



16 



MINING 
INDUSTRY iProduc 
COST 



Workmen s compensation, 

39.7%, and investigation 

cost, 0.3% 



Union 



benefits 
10.7 % 




nvestigation 
cost 2% 



FAMILY COST 



PUBLIC COST 



FIGURE 14.— Pie chart of roof-rib accident cost percent- 
ages, averages for 1980-84. 

million), and benefits, such as Social 
Security, union, and worker's compensa- 
tion ($95 million). 

COST PER ACCIDENT 

To obtain a more accurate approximation 
of accident costs, the program normalizes 



the data in the form of cost per acci- 
dent. Table 5 illustrates the wide gap 
that exists between the cost per accident 
of roof-rib fall accidents versus all 
other accident types. In 1984, the cost 
per roof-rib fall accident was estimated 
at $52,000 while the cost per accident of 
all other underground accidents was esti- 
mated at $16, 600. 

As table 5 shows, roof-rib accidents 
are over three times as costly as all 
other underground accidents. Specifical- 
ly, these large roof-rib costs can be 
traced to the cost of production losses 
(6.2 times more costly per accident), the 
cost of all benefits (1.3-4.1 times more 
costly per accident), and the cost of 
lost wages (3. 9 times more costly per 
accident). The higher production loss 
costs may be related to a longer shutdown 
period of the mine because of cleanup of 
the fallen material and resupport of the 
roof-rib fall area. The higher union, 
Federal, and workmen's compensation bene- 
fit costs and wage losses may be due to 
the greater severity of injuries caused 
by roof-rib fall accidents requiring a 
longer recuperation. Thus, the roof-rib 
accident victim loses more wages and sub- 
sequently increases the cost of the bene- 
fits that are paid by the mining industry 
and public sectors. Consequently, these 
large increases in cost per accident sub- 
stantiates the negative impact that 



TABLE 5. -Comparison of roof-rib accident costs and all other types of 
accidents for 1984, cost per accident 





Roof-rib 
accidents 


All other mining 
accidents 


Ratio 


Mining industry: 


$6,929 

10,917 

774 

2, 144 

63 


$5,322 

1,746 

611 

514 

13 


1.30 




6.25 




1.27 




4. 17 




4.85 




20,827 


8,206 


2.54 


Family: 

Lost wages (total cost to family).. 


21,324 


5,488 


3.88 


Public: 


9,714 
231 


2,860 
45 


3.40 




5.13 




9,945 


2,905 


3.42 




52,096 


16,599 


3.14 



Benefits. 



17 



roof-rib accidents have on various sec- 
tors of the economy. 

To evaluate cost trends over the 5-yr 
period, the ACIM program was modified 
such that all the costs were in 1983 dol- 
lars. This eliminated the effects of in- 
flation over the time period and allowed 
the data to be equitably compared. Fig- 
ure 15i4 shows that roof -rib accidents as 
well as all other types of accidents have 
erratically increased in cost (approxi- 
mately 30%) over the time period. There- 
fore, all types of accidents are becoming 
increasingly more expensive, even with 
the effects of inflation eliminated. 

COMPARISON OF ACCIDENT COSTS BASED ON 
DEGREE OF INJURY 

Figures 15B and 15C display the approx- 
imate cost ranges of roof-rib accidents 
with varying degrees of injury. Figures 
15B and 15C illustrates that the degree 




40 
30 
20 



o 


10 


o 


1,400 


ro 
O 


1,200 




1,000 


K 




Z 


800 


UJ 




Q 


600 


O 




O 


400 


< 






200 


a: 


8 


LU 




Q. 




1- 


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cn 




o 




o 


A 



KEY 

Roof- rib accidents 
All other accidents 



KEY 

Fatal 

Permanently disabled 



4 



2 - 



KEY 

1 Injured with days lost 
Injured with no days lost 




1980 



1981 



1982 



1983 



1984 



FIGURE 15.— Comparison of costs per accident. A, roof-rib 
accidents versus all other types of accidents (adjusted for in- 
flation); B, fatal roof-rib accidents versus permanent partial or 
total disabling accidents; C, roof-rib accidents resulting in 
days lost injuries versus no days lost injuries. 



of injury dramatically dictates the cost 
of a roof-rib accident (based on cost per 
accident in 1983 dollars). For instance, 
the cost per fatal accident (degree 1) is 
around $1 million per accident, but the 
cost of an accident resulting in a perma- 
nent partial or total disability (degree 
2) is around $300,000; while the cost of 
an accident resulting in lost workdays is 
close to $6,000, and the cost of an acci- 
dent with no lost workdays is about 
$500. 

For actual cost comparisons of various 
degrees of injury, roof-rib accident 
costs for 1983 were examined as shown in 
table 6. Comparison of the fatal acci- 
dents (degree 1) versus accidents result- 
ing in permanent disabilities (degree 2), 
which are the two most costly types of 
accidents, yields some interesting infor- 
mation. The estimated cost per accident 
resulting in a permanent disability is 
about 40% of the cost of a fatal acci- 
dent. The lower costs of degree 2 acci- 
dents result from considerably lower pro- 
duction losses, lost wages, and benefit 
costs (table 6). Whenever a fatal acci- 
dent occurs, it forces the whole mine to 
shutdown for one or more workdays, while 
with degree 2 accidents only the affected 
mine section may be closed. Consequent- 
ly, degree 2 accidents have lower produc- 
tion losses than occur with fatal 
accidents. 

Another comparison was made from table 
6, contrasting fatal accidents (degree 1) 
with accidents resulting in injuries with 
lost workdays (degrees 3-4) and injuries 
with no workdays lost (degrees 5-6), 
These comparisons are even more note- 
worthy. The degree 1 accident cost aver- 
aged $955, 000 per accident while the de- 
grees 3 and 4 accident cost averaged 
$6,123 per accident (150 times less than 
degree 1 accident costs) and the degrees 
5 and 6 accident cost averaged $517 per 
accident (1,800 times less than degree 1 
accident costs). 

The lower accident cost of degrees 3 
and 4 is due to zero production losses 
(more than likely a minuscule amount of 
production losses did occur), lower bene- 
fit costs, and less lost wages. With a 
less severe accident injury, the mine is 
not closed and therefore production 



18 



TABLE 6. - Comparison of costs per roof-rib accident based on degree of injury 
for 1983, cost per accident 



Cost factors 


Degree 1: 
Fatal 


Degree 2: 

Permanently 

disabled 


Degrees 3-4: 
injured with 
lost workdays 


Degrees 5-6: 

Injured with no 

lost workdays 


Mining industry: 


$126,668 

229,302 



55,117 

1,640 


$179,838 

9,415 

1,048 

22,124 




$3,023 



825 

32 










$517 

o 











412,727 


212,425 


3,880 


517 


Family: 

Lost wages (total family 


263,953 


85,023 


1,940 





Public: 

Social Security benefits'. 


273,301 
5,079 


103,329 



303 













278,380 


103,329 


303 





Total cost per accident 


955,060 


400,777 


6,123 


317 


Total accident cost 2 ... 


21,966,400 


3,606,998 


3,968,407 


62,535 



Benefits. 
2 Total estimated cost of roof -rib accidents for all degrees of injury in 1983 
$29,604,340. 



losses do not occur. Also, lost wages 
and benefit costs drop because of less 
severe injuries and shorter recuperation 
time. Costs of accidents with injuries 
of degrees 5 and 6 are reduced even fur- 
ther, almost to zero, except for the med- 
ical costs involved. Therefore, as the 
severity of the injury decreases the cost 
per accident decreases exponentially and 
considerably reduces the financial burden 
of the mining industry, the family, and 
the public. 



The total estimated cost of all roof- 
rib accidents in 1983, which was the low- 
est annual cost of the five years stud- 
ied, was calculated at approximately, 
$29,600,000 (table 6). Although these 
costs are only estimates, they are a mon- 
etary incentive for the mining industry 
to provide a safer underground environ- 
ment and to instill in its workers safe 
working habits for reducing the risk of 
roof-rib fall accidents. 



SUMMARY 



The effects of roof-rib accidents are 
extensive, ranging from the economic loss 
of equipment and production to the fatal 
and nonfatal injuries that result in 
lasting physical and financial impair- 
ments suffered by the victims and their 
families. Although roof-rib falls will 
probably never be totally eliminated, the 
statistics show that the following prob- 
lem areas need further attention: 



- High accident rates in Virginia, Colo- 
rado, and Utah. 

- Fairly high severity rates in Utah and 
Kentucky. 

- Increased risk of roof falls in August 
through October because of higher 
humidity. 



19 



- More accidents seem to occur in the 
second and third hour of each shift and 
after the lunch break. 

- High accident rates in mines with very 
thin coal seams (<36 in) and with very 
thick seams (>120 in). 

- High fatality rates in mines with very 
thin coal seams (<36 in). 

- High fatality rates at small mines with 
average annual work force of less than 
20. 

- Several worker activities seem more 
prone to accidents, such as operating 
continuous miners and roof bolting. 

- Increasing number of days away from 
work after a serious injury. 

Underlining the effects of roof-rib 
fall accidents are the accident costs, 
which have been estimated at $52,000 per 
roof-rib fall accident as opposed to 
$16,600 for all other types of accidents. 
Also, as the degree of injury resulting 
from a roof -rib accident becomes 



more severe, the cost of the accident 
increases exponentially. These high 
costs impact the mining industry, the in- 
jured workers and their families, and in- 
directly the public sector. 

These compiled roof-rib statistics also 
indicate the following encouraging areas 
that should be maintained: 

- U.S. roof -rib accident rates have con- 
sistently dropped in the last 20 yr, 
especially in recent years. 

- A lower probability of accidents exists 
in mines with coal seam thicknesses of 37 
to 120 in. 

- Mines employing more than 250 workers 
have a lower probability of accidents. 



- Longwall mining appears to be 
mining method. 



a safer 



While these statistics may not be abso- 
lute, they do offer a current profile of 
U.S. roof and rib accidents and related 
accident costs, which reinforces the need 
for continued ground control research. 



REFERENCES 



1. Peng, S. S. Coal Mine Ground Con- 
trol. Wiley, 1978, 200 pp. 

2. Thrush, P. W. A Dictionary of 
Mining, Mineral, and Related Terms. Bu- 
Mines Spec. Publ. , 1968, p. 151. 

3. U.S. Mine Safety and Health Admin- 
istration (Dep. Labor). Summary of Se- 
lected Injury Experience and Worktime for 
the Mining Industry in the United States, 
1931-1977. 1984, 69 pp. 

4. Schlick, D. P., R. Peluso, and 
K. Thirumalai. U.S. Coal Mining Acci- 
dents and Seam Thicknesses. Paper in 
Proceedings of Symposium on Thick Seam 
Mining by Underground Methods (Queens- 
land, Australia, 1976). Australasian 
Inst. Min. and Metall. , Symp. Ser. 
No. 14, Aug. 1976, pp. 61-74. 

5. U.S. Mine Safety and Health Admin- 
istration (Dep. Labor). Mine Injuries 
and Worktime, Quarterly: Closeout Edi- 
tions 1980-84. 119 pp. 



6. Spokes, E. M. New Look at Under- 
ground Coal Mine Safety. Min. Eng. (Lit- 



tleton, CO), v. 
pp. 266-270. 

7. Stateham, R. 
cliffe. Humidity: 
Coal Mine Roof 



26, No. 4, 1986, 



M. , and D. E. Rad- 
A Cyclic Effect in 
Stability. BuMines 
RI 8291, 1978, 19 pp. 

8. Haynes, C. D. Effects of Tempera- 
ture and Humidity Variations on Coal Mine 
Roof Stability. Paper in Proceedings 
of a Symposium on Underground Mining 
(Louisville, KY, Oct. 21-23, 1975). 
Natl. Coal Assoc. , Washington, DC, v. 2, 
1975, pp. 120-126. 

9. Theodore Barry and Associates. 
Accident Prediction Investigation Study 
(contract S0122023). BuMines OFR 38-73, 
1972, 174 pp.: NTIS PB 221000. 

10. U.S. Energy Information Adminis- 
tration (Dep. Energy). Coal Production 
1984. DOE/EIA-0118, 1985, 144 pp. 



10788 



95 



20 



11. U.S. Mine Safety and Health Admin- 
istration (Dep. Labor). Comparison of 
Injury Hazards in Different Coal Seams 
Heights. 1981, 22 pp. 

12. National Academy of Sciences. Fa- 
talities in Small Underground Coal Mines 
(contract J010014-5). BuMines OFR 124- 
83, 1983, 20 pp. 

13. National Research Council. Toward 
Safer Underground Coal Mines. Natl. 
Acad. Press, 1982, 190 pp. 

14. Loxley, T. E. , D. B. Lull, and 
J. 0. Rasor. Self-Advancing Roof Sup- 
ports for Longwall and Shortwall Mining, 
October 1974 Census, U.S. Naval Surface 
Weapons Center, Apr. 1975, 113 pp. 

15. Sprouls, M. W. Longwall Census 
'84. Coal Min. and Process., v. 21, Dec. 
1984, pp. 39-53. 

16. U.S. Energy Information Adminis- 
tration (Dep. Energy). Bituminous Coal 



and Lignite Productions and Mine Opera- 
tions. DOE/EIA-0118(78), 1980, 80 pp. 

17. Peake, C. V. Longwall Output Con- 
tinues to Rise. Coal Age, v. 91, No. 8, 
1986, pp. 58-60. 

18. Chi, D. N., and D. G. Di Canio. 
Mine Accident Cost Data Bases and Some 
Implications. Pres. at 1983 Am. Min. 
Congr. Min. Conv. , San Francisco, CA, 
Sept. 11-14, 1983, 5 pp., available from 
D.N. Chi, BuMines, Pittsburgh, PA. 

19. Di Canio, D. G. , and A. H. Nakata. 
Accident Cost Indicator Model To Estimate 
Costs to Industry and Society From Work- 
Related Injuries and Deaths in Under- 
ground Coal Mining, Volume I. Develop- 
ment and Application of Cost Model 
(contract J0255031, FMC Corp.). BuMines 
OFR 39(l)-77, 1976, 202 pp.; NTIS PB 264 
438. 



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